The ideas that have formed the scientific worldview of our generation are the product of two major intellectual revolutions, two significant reorientations in scientific thought. The first, associated with the names of Darwin, Wallace, Huxley, and Haeckel, spread the notion of evolution, of change, growth, and development, from its focus in biological investigation to swift domination of the entire climate of opinion of the age.

The second, carried through by the genius of Einstein, Planck, de Broglie, Heisenberg, and Schrodinger, introduced a novel set of fundamental concepts and principles into mathematical physics, and has puzzled our generation with the theory of relativity, quantum and wave mechanics, and the triumphs and mysteries of the structure of the atom…

Only recently have we begun to appreciate the real significance of the biological nature and setting of human life, and of taking time and temporal process seriously… The effect of the biological revolution of the last century was… [that] it placed man and his enterprises squarely in the setting of a natural environment, and gave them a natural origin and a natural history… Man has been transformed from a being supernaturally divorced from and elevated above the rest of nature, into a creature capable of interacting and cooperating with the other forces and resources in his natural environment, and in some measure bending them to his will.

The effect of the new physical revolution seems primarily humanistic: it emphasizes the human factor in scientific interpretations, and it points to a world in which human life can be a natural life. Not only has it underlined the genuine intellectual creation involved in scientific theory… It has definitely removed from the structure of science those basic assumptions and speculative generalizations of nineteenth-century thought which seemed most clearly to conflict with the demands of human life… Our electrical world of radiant energy is of a richness and complexity that does not seem so alien to the maze of human experience…

The shift of popular interest from biological to physical concepts in the last two decades is somewhat misleading. It is still doubtful whether the new physical theory, revolutionary as it has been for our notions of nature, will have anything like the impact on man and his position in the world that the biological revolution has already exerted. Its chief lesson may well be the reminder that it was not physics that created the world or human life, but the natural world that brought forth both physics and physicists.

Nor must it be thought that these successive intellectual revolutions in any sense cancel out their predecessors. In its ideas the scientific enterprise has been cumulative as well as original. The essential insights of the older views have been incorporated and reinterpreted but not discarded with each new advance. The seventeenth-century Order of Nature has been retained and pushed further in every field, even when profoundly modified, first by the historical and biological viewpoints of the nineteenth century, and then by the novel electrical conceptions of today…

It is physics and not biology that has for a generation been providing the spectacular new ideas. Yet it is still true that the idea of Evolution, of change, growth, and development, has been the most revolutionary notion in man’s thought about himself and his world in the last hundred years. This transformation of the setting of human life did not come about suddenly, overnight, it does not date from the justly epoch-making publication of Darwin’s Origin of Species in 1859. Rather that event symbolized the new attitude that had in many ways been making its progress in men’s thinking since the middle of the preceding century.

Darwin’s book, in fact, stands to our present-day scientific synthesis much as Newton’s Principia stood to the earlier mechanical synthesis, as the confident marshaling of evidence and the systematic formulation in strictly scientific terms of a view that had already been for some time gaining acceptance by the best intellects. Both the rationalistic thinkers of the Enlightenment, in their growing emphasis on progress, and the romantic reaction, in its singling out of a process of development in time as the fundamental fact in human experience, had paved the way for a successful biological formulation of Evolution. Only such a state of affairs can explain the almost instantaneous acceptance of Darwin’s doctrine when it was put forth in 1859…

This is not the place to enter into any detailed consideration of the progress of scientific discovery and theory; that fascinating story has been often told. But since it is beyond question the most important intellectual force in the last hundred years, it is worthwhile to present even a very inadequate summary of its significance. It was science, the mathematico-physical experimental learning of the seventeenth and eighteenth centuries, that really wrought the change from the intellectual world of the Middle Ages, changes that neither Renaissance nor Reformation had been able to bring about; and increasingly it has been the growth of scientific knowledge that has caused the steady spread of the naturalistic viewpoint in every field.

What the scientists learned from the romanticists, in a broader and more flexible outlook and method, in a wider conception of the extent of human experience, in a conviction of the fundamental importance of studying origins and development, has served only to entrench more strongly the scientific method and the scientific criterion of truth in the minds of all educated men...

Our present-day Order of Nature may be far more intricate [than that of the eighteenth century], but it is also far more comprehensive and far more solidly established than ever before. It is hardly surprising that this revolution in physical theory and concepts has provoked an immense amount of philosophizing, both about the new pictures of the world suggested, and about the very nature of the scientific enterprise itself. Both philosophers and scientists have undertaken a careful and critical analysis of the function and nature of scientific theory in general, and of the mathematical formulations of physical theory in particular.

The older view that Newtonian science was a direct reading of the structure of nature is no longer tenable. Scientific theory and concepts, it is only too apparent, develop and change in time; and he would be hardy today who maintained that any of the present ideas express “the way things really are.” The primary function of theory and hypothesis, it is now clear, is to organize discoveries already made and suggest new questions to put to nature.